The present invention relates to a vehicle steering system with auxiliary steering assist, and more specifically to a vehicle steering system with hydraulic power assist.
A known hydraulic power steering system includes a control valve with an inner valve member which is coaxial with and rotatable relative to an outer valve member or sleeve. To effect actuation of a hydraulic steering assist motor to turn the steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member against the influence of a torsion bar. It is known to increase the resistance necessary to actuate the power steering system as vehicle velocity increases by having the fluid pressure in a reaction chamber increase as the vehicle velocity increases. Increasing the fluid pressure in the reaction chamber increases the resistance to relative rotation between the first and second valve members which control fluid flow to a power steering motor and provides the operator with a better "feel" of the road and driving conditions and improves safety. Therefore, as vehicle velocity increases the steering feels more like a manual steering system.
It is also known to include a power steering pump for supplying pressurized fluid to a power steering system, which pump turns on only when the velocity of the vehicle is below a predetermined value. When the velocity of the vehicle exceeds the predetermined value, an input shaft connected to a steering wheel is directly connected to an output member which is connected to a pinion of a rack and pinion steering system. When the velocity of the vehicle exceeds the predetermined value, the steering becomes manual and when the velocity of the vehicle is below the predetermined value there is power assisted steering.